Experimental and numerical investigation on meso-fracture behavior of Beishan granite subjected to grain size heterogeneity

Research on crack evolution mechanism and failure pattern of Beishan granite is vital for evaluation of the engineering stability of disposal repository. The mechanical properties, especially the strong heterogeneity, significantly affect the cracking behavior of granite. To investigate the fracture behavior of heterogeneous granite, mesoscopic three-point bending tests were conducted on Beishan granite using an in situ scanning electron microscope (SEM) with a loading device, and the entire cracking process was observed in real time. Additionally, the effect of grain size heterogeneity on strength and deformation behavior of granite was quantitatively investigated by introducing a statistically defined heterogeneity index into the grain-based model in two-dimensional Particle Flow Code (PFC-GBM), and the crack development was characterized through fractal theory. The results demonstrated that fracture mechanism of granite is mainly tensile and shear failure, and the fracture surface of tensile failure is rough and irregular, which is mainly caused by the tearing between various mineral crystals, whereas the fracture surface of shear failure is relatively smooth, which is mainly caused by the dislocation and slip between minerals. The crack edge can be regarded as the damage zone, and the development of the main crack is promoted by the continuous generation and combination of micro-damage at the crack tip. The crack propagation path from a mesoscale perspective is a process of continuous crack orientation, which depends on the competition mechanism between mineral properties and maximum tensile stress. Generally, the granite strength tends to increase with the increase of heterogeneity.